Field of the Invention
The invention relates to a method for contacting a semiconductor configuration.
The invention relates, in particular, to a semiconductor configuration which is composed of a given polytype of silicon carbide at least in specific semiconductor regions, in particular the semiconductor regions which are to be contacted. The semiconductor regions that are to be contacted are in particular p-conducting.
Silicon carbide (SiC) in monocrystalline form is a semiconductor material having outstanding physical properties that seem to make this semiconductor material interesting particularly for power electronics, even for applications in the kV range, inter alia due to its high breakdown field strength and its good thermal conductivity. Since the commercial availability of monocrystalline substrate wafers, especially ones made of 6H and 4H silicon carbide polytypes, has risen, silicon carbide-based power semiconductor components, such as e.g. Schottky diodes, are now also receiving more and more attention. Other silicon carbide components which are becoming increasingly widespread are pn diodes and transistors such as, for example, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors).
Stable ohmic contacts to semiconductor regions of different conduction types are indispensable for the functioning of these components. In this case, the lowest possible contact resistances are sought in order to minimize undesirable losses at the semiconductor-metal junction.
The overview paper xe2x80x9cOhmic contacts to SiCxe2x80x9d by G. L. Harris et al. from xe2x80x9cProperties of Silicon Carbidexe2x80x9d ed. by G. L. Harris INSPEC, 1995, pages 231-234 contains a summary of contacting methods for silicon carbide of different polytypes and conduction types. With regard to the contact-connection of p-conducting SiC, the overview paper and the cross-references cited reveal the current state of the art generally accepted by experts, which is outlined below:
Aluminum is predominately used for contact-connecting p-conducting SiC. Since aluminum is soluble in small amounts in SiC and acts as an acceptor, a zone highly doped with aluminum can be produced in a boundary region between the aluminum-containing contact region and the semiconductor region made of SiC. In order to avoid evaporation of the aluminum, which melts at a temperature as low as 659xc2x0 C., during a subsequent thermal treatment for forming the ohmic contact, at least one covering layer made of a material having a higher melting point, such as e.g. nickel, tungsten, titanium or tantalum, is applied to the aluminum.
In order to be able to exploit the advantageous contact properties of a specific contact material on p-conducting SiC, these methods of the prior art consequently require a second layer which protects the underlying first layer. The first and second layers are applied by separate, successive technological process steps with different materials in each case.
The paper xe2x80x9cTitanium and Aluminium-Titanium Ohmic Contacts to p-Type SiCxe2x80x9d, Solid-State Electronics, Vol. 41, No. 11, 1997, pages 1725-1729, discloses an aluminum-titanium alloy as material for an ohmic contact on p-conducting SiC. The alloy used in this case has a proportion by weight of 90% for aluminum and correspondingly a portion by weight of 10% for titanium. The contact made of the aluminum-titanium alloy is not covered with a further layer. The paper reports on problems with reproducibility and with very thin contact layers. With thin contact layers, in particular, the aluminum may volatilize from the aluminum-titanium alloy.
The paper xe2x80x9cOhmic Contacts to p-type SiC with improved thermal stabilityxe2x80x9d, by Liu, S., et al., 7th International Conference on Silicon Carbide, III-Nitrides and Related Materials Stockholm, September 1997 describes an ohmic contact on p-conducting SiC which is produced through the use of a layer structure including an aluminum layer, a nickel layer and a tungsten layer. After this layer structure has been applied, a heat-treatment process is carried out, in the course of which, inter alia, the aluminum and the nickel are mixed together, but a material composition that is homogeneous over the depth of the contact region is not formed. Moreover, this mixing also takes place only in the presence of the third layer made of tungsten. During the heat-treatment process, a considerable proportion of tungsten in turn diffuses from this third layer right into the boundary region of the ohmic contact and of the p-conducting SiC. In this case, however, the tungsten can then lead to impairment of the ohmic contact behavior.
It is accordingly an object of the invention to provide a method for contacting a semiconductor configuration which overcomes the above-mentioned disadvantages of the heretofore-known semiconductor configurations and methods of this general type and which provide an improved contact-connection of p-conducting SiC in comparison with conventional configurations and methods. In this case, the contact on the p-conducting semiconductor region should have a low contact resistance and should be thermally stable.
With the foregoing and other objects in view there is provided, in accordance with the invention, a semiconductor configuration with an ohmic contact-connection, including:
at least one p-conducting semiconductor region composed of silicon and carbon in a form of silicon carbide;
at least one p-type contact region adjoining the at least one p-conducting semiconductor region and composed of a material having nickel as a first material component and aluminum as a second material component, the at least one p-type contact region having a substantially uniform material composition throughout; and
a boundary region extending into the at least one p-conducting semiconductor region and into the at least one p-type contact region, the boundary region being composed substantially exclusively of the silicon and the carbon of the at least one p-conducting semiconductor region and of the nickel and the aluminum of the at least one p-type contact region.
In other words, the semiconductor configuration according to the invention includes:
a) at least one p-conducting semiconductor region made of silicon carbide and
b) at least one p-type contact region adjoining the p-conducting semiconductor region, in which case
c) the p-type contact region is composed of a material having nickel as a first material component and aluminum as a second material component,
d) an approximately identical or uniform material composition is present in the p-type contact region, and
e) practically exclusively the silicon and the carbon of the p-conducting semiconductor region and the nickel and the aluminum of the p-type contact region are present in a boundary region, which extends into the p-conducting semiconductor region and into the p-type contact region.
With the objects of the invention in view there is also provided, a method for contacting a semiconductor configuration, the method includes the steps of:
providing at least one p-conducting semiconductor region formed of silicon carbide; and
applying a material having nickel as a first material component and aluminum as a second material component on the at least one p-conducting semiconductor region for forming at least one substantially homogeneous p-type contact region on the at least one p-conducting semiconductor region, by simultaneously applying both, the first material component and the second material component such that a given mixture ratio of the first material component and the second material component is established at an interface between the at least one p-conducting semiconductor region and the at least one p-type contact region prior to a heat-treatment process.
In other words, the method according to the invention includes:
a) at least one substantially homogeneous p-type contact region is formed on at least one p-conducting semiconductor region made of silicon carbide by
b) a material having nickel and aluminum as a first and a second material component, respectively, being applied, for providing the p-type contact region, in which case
c) both material components are applied simultaneously, such that a predetermined mixture ratio of both material components is already established at an interface between the p-conducting semiconductor region and the p-type contact region prior to a heat-treatment process.
The invention is based on the insight that, contrary to the customary procedure employed by experts, a stable and reproducible ohmic contact-connection of p-conducting silicon carbide can be effected not only by successively applying two or more layers each of a different material, but rather also through the use of a single layer having an approximately identical, i.e. homogeneous material composition. The relevant p-type contact region is composed of a material containing the materials used for the individual layers in the prior art, namely nickel and aluminum, as first and second material components, respectively. In this case, the material may be present in the form of a mixture, a batch, an alloy or a compound of these two material components.
Prior to the start of the heat-treatment process, a quaternary material system including the individual components silicon, carbon, nickel and aluminum is already present at the interface with SiC. This has a particularly favorable effect on the ohmic contact formation during the heat-treatment process. A desired mixture ratio between the aluminum and the nickel in particular at the interface with the p-conducting SiC can already be established without difficulty during the simultaneous application of the two material components before the heat-treatment process. Since a further covering or protective layer made of another material is not provided on the contact region, the four above-mentioned elements of the quaternary material system are present practically exclusively, i.e. apart from unavoidable contaminants and the dopant atoms of the p-conducting semiconductor region, in the boundary region even after the heat treatment. Possible impairment of the contact resistance as in the case of the prior art does not occur, therefore, since the boundary region is practically free of undesirable impurity atoms.
Compared with the simultaneous application of both material components, successive application of a first layer of aluminum and a second layer of nickel would be less favorable since, during the subsequent heat-treatment process, the two material components would first have to be mixed together to a sufficient extent in order to be available equally at the interface. Consequently, in the case of layer-by-layer application of the two material components, there would still not be a quaternary systemxe2x80x94which is particularly favorable for ohmic contact formationxe2x80x94at the interface between the contact layer and the semiconductor region at the beginning of the heat-treatment process. By contrast, simultaneous application of the two material components also additionally simplifies the fabrication process since a separate process step for a second layer is obviated.
Moreover, a material made of nickel and aluminum affords the advantage over the aluminum-titanium alloy used in the prior art that nickel forms a silicide when heated above 300-400xc2x0 C., for example during the forming operation, in contrast to the carbide-forming titanium. As a result, a silicon position becomes free in the SiC microstructure of the p-conducting semiconductor region, to which position aluminum can be bound as an acceptor. In contrast to this, however, aluminum cannot be bound as an acceptor, into the crystal microstructure, to a carbon position that becomes free on account of the carbiding of titanium. Therefore, and also due to the higher thermal stability of nickel compared with titanium, a stable contact with a low contact resistance results in the case of a material which is composed of nickel and aluminum and used for the p-type contact region.
After the heat-treatment process, a slight deviation from the material homogeneity is established within the p-type contact region. This slight inhomogeneity stems from exchange processes between the materials in the boundary region of the p-type contact region and of the p-conducting semiconductor region.
Thus, in the boundary region, for example, the aluminum of the applied material will migrate to a certain extent into the p-conducting semiconductor region, where it is bound as an acceptor at the corresponding lattice locations. A shift of the material composition likewise results in the boundary region since the material of the p-type contact region with the nickel contains, as already mentioned, a siliciding material component. Silicon from the p-conducting semiconductor region is consequently mixed together with the nickel of the p-type contact region and forms a corresponding silicide.
However, the region in which the above-described mixing-together processes take place does not extend right into the depth of the p-type contact region, so that the above-mentioned differences in the material composition are primarily produced only in the boundary region. The expression xe2x80x9csubstantially uniform material compositionxe2x80x9d is to be understood in this sense.
Moreover, differences in the material composition which are to be attributed to customary contaminants in starting substances are likewise regarded as non-critical here. Such contaminants may be present in a proportion by volume of at most 10xe2x88x923, often even just of at most 10xe2x88x926.
In one advantageous embodiment, the aluminum is present with a proportion by volume of from 0.1 to 50% in the material. A proportion of from 20 to 50% is particularly preferred here since the contact resistance which can be achieved in this region continuously improves with an increasing proportion of aluminum. It has been shown that the volatilization of the aluminum can be distinctly reduced and even suppressed if the proportion by volume of aluminum is chosen to be less than 50%. In this case, the proportion of nickel in the material, which is then correspondingly greater than 50%, prevents, in particular at rising temperatures, the formation of liquid aluminum islands and the resulting undesirable evaporation of aluminum. Consequently, choosing the proportions of nickel and aluminum within the limits specified additionally increases the stability. This is beneficial particularly during the heat-treatment process which is advantageously carried out for the purpose of forming the ohmic contact, but also if the semiconductor configuration is provided for an application in the high-temperature range. This is a preferred application for an SiC component on account of the outstanding properties of SiC in this regard.
To form a good ohmic contact, it is advantageous if the p-conducting semiconductor region has a sufficiently high dopant concentration at least at the interface between the semiconductor region and the p-type contact region. In this case, the dopant concentration preferably lies between 1017 cmxe2x88x923 and 1020 cmxe2x88x923. A particularly good contact results if the dopant concentration is at least 1019 cmxe2x88x923.
Other embodiments of the method relate to the application of the material to the p-conducting semiconductor region.
In one embodiment of the method, the material which is applied to the two semiconductor regions is taken from at least two separate sources. In this case, the sources each contain at least one material component, in particular the first or the second material component. They are withdrawn from the sources by simultaneous vaporization or sputtering. The p-type contact region is subsequently formed by depositing the material components on the p-conducting semiconductor region. In this case, the material for the p-type contact region is produced either while still in the vapor phase from the individual material components or in the course of the deposition process. The process parameters can ensure that a specific intended mixture ratio is adhered to.
By contrast, an alternative embodiment provides for a source material firstly to be prepared from the first and second material components and then to be sputtered in a second method step. The released particles of the material form the p-type contact region on the p-conducting silicon carbide, as in the previously described embodiment.
In an advantageous embodiment, the semiconductor configuration is subjected to a brief heat-treatment process after the p-type contact region has been applied. In this case, the semiconductor configuration is preferably heated to a maximum temperature of at least 500xc2x0 C., in particular of about 1000xc2x0 C., and then held at about this maximum temperature for up to 2 hours, in particular for 2 minutes. However, the heat-treatment process may also include only a heating phase and an immediately following cooling phase, without a hold time at a maximum temperature being provided in between. This process serves for forming the p-type contact region. It has been found that a thermally stable contact with good ohmic characteristics and a low contact resistance results on the p-conducting SiC after this heat-treatment process.
The p-conducting semiconductor region that is to be contact-connected may include different SiC polytypes. There are embodiments in which SiC in the form of 6H, 4H, 15R or 3C SiC is used for the p-conducting semiconductor region. However, other polytypes are likewise possible.
Outside the p-conducting semiconductor region, the semiconductor configuration may also be composed, at least in regions, of a material other than SiC. Therefore, one embodiment provides at least one further semiconductor region, for example a substrate, made of a different material than SiC, for example made of silicon (Si), gallium arsenide (GaAs) or gallium nitride (GaN). This substrate is then integrated at least with the p-conducting semiconductor region made of Sic to form a hybrid semiconductor configuration.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for contact-connecting a semiconductor configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.